1. Field of the Invention
The present invention relates to an arrangement for DPCM coding of television signals in which respective estimated values are subtracted from digitized picture elements and the difference signals obtained are used for signal transmission after quantization and coding and in which a recursive signal path that contains a first adder for the formation of reconstructive picture element signals from the quantized difference signals and the estimated values includes a limiter, a predictor for the formation of estimated values and a subtractor for the formation of the different signals.
2. Description of the Prior Art
An arrangement of the type generally set forth above is disclosed, for example, in the Proc. IEEE, Vol. 73, No. 4, April, 1985, pp. 592-598, with particular reference to FIGS. 1, 2 and 4, and is basically illustrated in the present FIG. 1. In FIG. 1, a sequence of digitized picture element signals s is received at an input 1, the picture element signal s is therefor supplied from therefor sample and hold stages that are not shown. Efforts are undertaken for the reduction of the data flow to remove redundant and irrelevant parts of the picture signal in order, for example, to be able to lower the bit transmission rate without thereby deteriorating the picture quality. More specifically, this occurs in that it is not the successive picture element signals that are transmitted via the transmission channel leading to a receiving location, but only the difference signals that are formed by the formation of the difference between a respectively current picture element signal s and an estimated value s identified in a predictor on the basis of the preceding picture element signals. Such a method is also referred to as difference pulse code modulation (DPCM).
The formation of the difference signal required for DPCM coding occurs in a subtracter 2 whose first input is connected to the input 1 and whose second input is connected to a predictor 3. Every difference signal .DELTA. (also referred to as an estimated error) is quantized in a quantizer 4, whereby resulting quantized difference signal .DELTA..sub.q =.DELTA.+q affected with the quantization error q is coded in a coder 5 and is supplied to the transmission channel via an output 6. A recursive signal path is provided for forming the estimated value s. This signal path extends from a circuit point 7 at the output side of the quantizer 4, includes a first adder 8, a limiter 9 and the predictor 3. Such signal path terminates at the second input of the subtracter 2. The output of the predictor 3 is also connected to a second input of the first adder 8 that forms what is referred to as a reconstructed picture element signal s.sub.R by addition of the quantized different signal .DELTA..sub.q and of the estimated value s. The predictor 3 supplies the estimated value s from at least one of the preceding picture element signals for each current picture signal s.
When, according to FIG. 2, the current picture element lying in the line n in a television picture m is referenced X, the picture element scanned immediately therebefore is referenced A, the picture element of the preceding line n-1 corresponding to X is referenced C, and the picture elements adjacent to the latter and scanned immediately preceding or, respectively, following the latter are referenced B and D and when, furthermore, the corresponding picture elements of the preceding picture m-1 are referenced X' and A -D', then the following occurs. The picture element signals of at least one of the points A-D can be utilized for the formation of the estimated value s for the picture element signal X, whereby one speaks of a two-dimensional (2D) prediction. When the picture element signals of at least one of the picture elements X' and A'-D are additionally more exclusively used for this purpose, then a three-dimensional (3D) prediction is present. In the former instance, the estimated value s, for example, be calculated according to the 2D-estimating equation EQU s=.alpha..multidot.s.sub.A +.beta.s.sub.B +.gamma..multidot.s.sub.C +.delta..multidot.s.sub.D ( 1)
and, in the latter instance, can be calculated, for example, according to the 3D estimating equation EQU s=s.sub.X' ( 2)
whereby s.sub.A references the reconstructed picture element signal of the picture element A, s.sub.B represents that of the picture element B, etc., and whereby the coefficients .alpha., .beta., .gamma., and .delta., represent weighting factors that are assigned to the individual picture element signals. The estimating equation (2) is recommended when the contrast of the current picture element X, also referred to as "activity", is low in comparison to the picture elements surrounding the current picture element X.